Power supplies of various types are widely used in electronics and can be found in literally any electronic device. Many of these power supplies produce a high voltage output and are capable of driving hazardous voltages and current. Unless special provision is made, these power supplies can retain a large voltage in their output filter capacitors even when the power supply is turned off or the output load is removed. The energy stored in the output filter capacitors is given by the expression
                              Energy          =                                    CV              2                        2                          ,                                  ⁢        where                                          C          =                      the            ⁢                                                  ⁢            capacitance                          ,                                  ⁢        and                                V        =                  the          ⁢                                          ⁢          output          ⁢                                          ⁢                      voltage            .                              
This energy is typically measured in tens of joules and can reside on unloaded electrolytic filter capacitors for hours or even days. This large residual charge at high voltage poses a significant hazard to service and operating personnel, as well as to the power supply itself and associated equipment.
One approach to resolving this problem is shown in the schematic diagram of FIG. 1, where a 120V AC input is provided via a switch 72 and power transformer 74 to a rectifying bridge 76 in a DC power supply 70. The output filter capacitors are shown in simplified form as capacitor 78 connected across the power supply's output terminals 82a and 82b. In this approach, a bleed resistor 80 is also connected across the output terminals 82a, 82b for dissipating the residual charge on the output capacitors. The primary problem with this approach is the presence of the bleed resistor 80 in the circuit during operation of the DC power supply 70, resulting in substantial energy dissipation via the bleed resistor.
Another approach to discharging the output filter capacitors of a DC power supply 90 is shown in FIG. 2. In this approach, the otherwise-unused off-throw contacts 94a and 94b of a double pole double throw (DPDT) on/off power switch 92 are used to discharge the output filter capacitors, which are shown in FIG. 2 as a single capacitor 100 for simplicity. An input current is provided via the DPDT on/off power switch 92 to a power transformer 102, the output of which is rectified by a bridge 104. When the DPDT on/off power switch 92 is moved to the off position, it establishes a discharge path 96 through first and second resistors 98a, 98b and the primary winding 102a of the power transformer 102. The energy stored in the output filter capacitors 100 is rapidly dumped. In this approach, the first and second resistors 98a, 98b in the turn-off discharge path are only in the circuit when the DC power supply 90 is turned off, and thus do not reduce the efficiency of the power supply during operation. However, this approach requires a complicated switching arrangement at the input of the DC power supply. The approaches to discharging the power supply output filter capacitors of FIGS. 1 and 2 are described in the Jul. 5, 2001 edition of Electronic Design News, in an article entitled “Quickly Discharge Power-Supply Capacitors”, by Stephen Woodward, page 132.
Other approaches to dissipating the charge on the output filter capacitors employ a manually operated switch for discharging this energy when the converter is turned off or the output load is removed. This latter approach is, of course, not automatic. Other approaches are automatic in operation, but require additional circuitry in the power supply, resulting in a more complicated arrangement and require circuitry for interfacing the power supply with the energy discharge circuit. Moreover, modern high power supply modules are hot unpluggable and therefore have no mechanical power switches.
The present invention addresses the aforementioned limitations of the prior art by providing a power supply with a device, which rapidly and automatically provides for the full discharge of energy stored in the power supply's output filter capacitors. The device includes a combination of a switching transistor and bleed resistor which are not in circuit during normal operation of the power supply, but are automatically switched in circuit when the power supply input is turned off and the output load is removed from the converter to fully discharge the output filter capacitors. The discharge circuit is integral with the power supply and does not itself reduce the efficiency of the converter during normal power supply operation. While disclosed primarily in terms of use in a soft switching power supply, i.e., where switching occurs at essentially zero voltage, the present invention is applicable for use in any type of switching and linear power supply.